7/27/2019 Experiment 6 Transformer Voltage Regulation and Efficiency

1/4

Purdue University CalumetDepartment of Engineeri ng Technology

Electrical Power and Machinery ECET-212

Experiment 6

Transformer Voltage Regulation and Efficiency

Objectives:

1. To calculate and measure the voltage regulation of a transformer2. To calculate and measure the efficiency for a transformer

Equipment and Supplies:

1- 0.25 KVA transformer1- Wattmeter1- AC Voltmeter1- AC Ammeter1- Resistive load box (RL100A)1- Line cord1- Variac

Explanation:

TRANSFORMER VOLTAGE REGULATION AND EFFICIENCY

When a transformer is loaded with a constant primary voltage, then the sec terminal voltage drops

(Assuming a unity or lagging power factor, it will increase if power factor is leading) because of itsinternal resistance and leakage reactance.

Let V2n1 = secondary terminal voltage at no-load

V2f1 = secondary terminal voltage at full-load

The change in secondary terminal voltage from no-load to full load is =V2n1-V2f1. This change divided

by V2n1 is known as regulation down. If this change is divided by V2f1, then it is called regulation

up.

Therefore,

% regulation down = V2n1-V2f1 *100

V2n1

and

%regulation up = V2n1-V2f1 * 100

7/27/2019 Experiment 6 Transformer Voltage Regulation and Efficiency

2/4

V2f1

In further treatment, unless stated otherwise, regulation is to be taken as regulation down. The lower the

regulation the better the transformer, because a good transformer should keep its secondary terminal

voltage as constant as possible under all conditions of load.

LOSSES IN A TRANSFORMER

(i) Core or iron loss: It includes both hysteresis loss and eddy current loss. Because the core fluxin a transformer remains practically constant for all loads (its variation being 1 to 3 % from

no-load to full load). Hence the core loss is practically the same at all loads.

These losses are minimized by using steel of high silicon content for the core and by using

very thin laminations. Core iron is found from the open-circuit test. The input of the

transformer when on no load measure the core loss.

(ii) Copper loss: This loss is due to the ohmic resistance of the transformer windings. Totalcopper loss = I1

2

R1 + I22

R2. It is clear that copper loss is proportional to (current)2

orkVA2. In other words, copper loss at half the full-load is one-fourth of that at full-load.

The value of copper loss is found from the short-circuit test.

EFFICIENCY OF A TRANSFORMER:

The efficiency of a transformer at a particular load and power factor is defined as the output divided by

the input.

Efficiency = Output

Input

But a transformer being a highly efficient piece of equipment, has a very small loss, hence it is

impractical to try to measure eff by measuring input and output. A better method is to determine the

losses and then calculate the efficiency from

Efficiency = Output

Output + losses

Output

Output + copper loss + core loss

It may be noted here that efficiency is based on power output in watts and not in volt-amperes, although

losses are proportional to VA. Hence at any volt-ampere load, the eff depends on power factor, being

maximum at a power factor of unity.

Efficiency = V2 I2 cos

V2 I2 cos + copper loss + core loss

7/27/2019 Experiment 6 Transformer Voltage Regulation and Efficiency

3/4

PROCEDURE:

VOLTAGE REGULATION

1. Calculate the % regulation for the 0.25KVA transformer when loaded to rated conditions use thevalue of Rp and Xp measured in the last laboratory experiment..

2. Set up the circuit as in Fig. 1.3. Energize the transformer and adjust the load for rated current.

NOTE: To avoid overheating do no energize for long periods of time.

4. Measure the secondary voltage under full load.5. Disconnect the load resistor from the circuit and measure the secondary voltage with no load.6. Calculate the percent regulation and compare it to the theoretical regulation.

TRANSFORMER EFFICIENCY

1. Calculate the efficiency for rated unity power factor load. Use the measured values of the coreand copper losses from the last laboratory experiment.

2. Set up the circuit of Fig. 2.3. Set the load for rated load current.4. Measure the input power.5. Calculate the output power from the measured values of secondary voltage and current.6. Calculate the efficiency for the transformer as power out over power in.7. Compare the measured and theoretical results.

7/27/2019 Experiment 6 Transformer Voltage Regulation and Efficiency

4/4

8. Load the transformer in steps from 0 to 100% of rated current. Measure the output voltage andcurrent.

9. For each of the load do the following:1. Calculate the power output = V2 I2cos 2. Calculate copper loss = I22 Rs3. Calculate the efficiency = Power Output

Power output + Pcore + P copper

4. Make a plot of copper loss, core loss and efficiency against the %load on the same co-ordinate system.

5. Locate the peak efficiency point and verify that this point occurs when copper loss equalscore loss.

EXTRA CREDIT:

Write a computer program to do the above steps.

= P out = V2I2cos

P out + P losses V2I2cos + P core + I22Rs